1. Field of the Invention
This invention relates generally to high speed logic circuitry and, more particularly, to high speed transistor-transistor logic (TTL) circuits having improved output circuitry.
2. Description of the Prior Art
Prior art TTL logic circuits include first and second current drive transistors coupled between a source of binary input logic signals and first and second output transistors coupled in a push/pull configuration. The output devices are turned on and off depending upon the state of the binary logic input signals.
The output terminal of the TTL circuit is generally coupled to the junction of the first and second output transistor; i.e. the junction of the emitter of the pull-up transistor and the collector of the pull-down transistor. The output terminal may assume one of two distinct logic levels. When the pull-up transistor is in or near saturation and the pull-down transistor is off, the output terminal will rise to a binary one level. If, the pull-up transistor is off and the pull-down is in or near saturation, the output terminal will be at a binary zero level.
A problem associated with the prior art circuit manifests itself when the circuit is used to drive a highly reflective load. That is, voltage excursions may be induced in the output due to negative current reflections causing the pull-up transistor to turn off. Ringing or oscillation may occur if the pull-up transistor is in or near saturation causing the output terminal to be at a binary one level when the negative reflection appears. The pull-up transistor will be turned off due to the reflection; however, when the negative reflection terminates, the pull-up transistor will again turn on causing the output to go high which in turn may cause another reflection which in turn will cause the pull-up transistor to turn off, etc.